Bill McKibben: In July 1893, 115 years ago, the historian Frederick Jackson Turner told an academic symposium that the American frontier was closed -- a shocking notion for a people who'd defined themselves by their steady expansion across the continent. This spring, something just as profound and defining has happened: Pulled back by the inescapable gravity of higher prices and the growing scarcity of fossil fuels, we're starting a slow recoil into more dense and compact regions and localities. The frontier of endless mobility that we've known our entire lives is closing.
